The present invention claims priority to Japanese Patent Document No. 2000-375044 filed on Dec. 8, 2000, the discloser of which is incorporated herein by reference.
The present invention relates to an arc electrode structure for synthesis of carbon nanostructures, and a method for producing carbon nanostructures therewith. More particularly, the invention relates to an arrangement of electrodes for producing an arc-plasma discharge to synthesize carbon nanostructures by consumption of carbon-containing electrodes, or by a chemical vapor deposition (CVD) process. Carbon nanostructures that may be produced include for example, single wall nanotubes (SWNTs), multi-wall nanotubes (MWNTs), fullerenes, endohedral metallofullerenes, carbon nanofibers, and other carbon-containing nano-materials.
Carbon nanostructures are generally known to be produced by arc-discharge between one anode and one cathode. See, for example: Japanese 11-263609, published Sep. 28, 1999; xe2x80x9cGrowth and Sintering of Fullerene Nanotubesxe2x80x9d by D. T. Colbert et al., Science Magazine, vol. 266, Nov. 18, 1994; xe2x80x9cFullerene Productionxe2x80x9d by Lowell D. Lamb et al., Journal of Phys. Chem. Solids, vol. 54, No. 12, pp 1635-143, Elsevier Science, Ltd. Great Britain, 1993; and U.S. Pat. No. 6,063,243. Because only one anode and one cathode are used, a limited arc-plasma region is obtainable. Further, the electrodes include flat surfaces which oppose one another. Because only flat electrode surfaces oppose one another, it is difficult, if not impossible, to control the direction and region of arc-plasma. Consequently, it is difficult to control the final carbon nanostructure produced. Further, the area outside of the arc-plasma region quickly drops in temperature. Due to the limited size of the arc-plasma region, and due to the low temperature outside of the arc-plasma region, the reaction species are quenched quickly, not heat annealed. Such quick quenching of the reaction species leads to a high production of amorphous carbon and other unwanted species, resulting in a low yield of carbon nanostructures. Therefore, only short SWNTs may be produced by these apparatuses and methods.
Typically anodes are carbon rods having catalyst mixed therein. Catalysts having a low boiling or sublimation point easily run out of the hot electrodes and, therefore, are not fully utilized.
During soot generation, soot is generally deposited on the inner walls of the arc electrode chamber and, thus, must be harvested. As noted in xe2x80x9cFullerene Productionxe2x80x9d by Lamb et al., harvesting soot presents real health risks. Therefore, soot harvesting must be done carefully which typically means slowly and at a large expense. Therefore, soot harvestation is tough work, especially in big chambers.
Lastly, in traditional arc-CVD apparatuses, organic vapor is introduced through an inlet other than the center of the electrode. That is, in conventional structures, gaseous reaction species are introduced to the side of an arc-plasma discharge region. See xe2x80x9cMass production of single-wall carbon nanotubes by the arc plasma jet methodxe2x80x9d, by Ando et al., Chemical Physics Letters 323, Elsevier Science B.V., Jun. 23, 2000. Therefore, the organic vapor is not preheated by the hot electrode, and is not introduced into the arc plasma region completely and evenly, which results in a low yield of SWNTs. Also, neither of the electrodes are cooled with the flowing organic vapor. Moreover, because the organic vapor passes by the side of the arc-plasma region, there is both a considerable amount of unused organic vapor, and a considerable portion of the arc-plasma region that is under-used.
In another typical CVD apparatus, gas is passed through a rotating tube heated by a furnace. In order to keep the tube from melting, however, this process can only be performed at about 1000xc2x0 C. Therefore, due to this temperature limitation, a large amountxe2x80x94up to about 90%xe2x80x94of the gas is unused or wasted. Accordingly, this process has a very low efficiency.
The present invention relates to improved arc electrode structures, and related apparatusses. For example, the present invention relates to an arc electrode structure for efficiently producing carbon nanostructures and, in particular, SWNTs, wherein the yield of SWNTs is increased.
The present invention allows the direction and region of arc plasma to be adjusted so that the final product is controllable. That is, because the present invention in an embodiment, includes an annular electrode having a sloped surface, the direction and region of arc-plasma easily can be adjusted. Additionally, because the electrode includes a sloped surface, it is automatically cleaned. That is, deposits, that would have otherwise collected on a flat electrode surface, slide off of the sloped surface of the present invention""s electrode, thereby cleaning the electrode surface. Further, the sloped surface of the electrode, in an embodiment, includes a plurality of holes therein for holding catalyst, even as it reaches its boiling or sublimation point. The holes have varying depths so that catalyst is continuously, and uniformly, distributed throughout the arc-plasma region during the entire duration of arc-discharge.
In addition to a sloped surface on one of the electrodes, the present invention in an embodiment, includes a plurality of second electrodes disposed in opposition to the first-electrode""s sloped surface. The provision of at least two second electrodes contributes to the adjustability of the direction and region of arc-plasma. The second electrodes are positioned so that their arcs combine to produce a larger, hotter, arc-plasma region which leads to a longer reaction time. The longer reaction time, in turn, results in longer SWNTs, and an increased yield thereof.
In a further embodiment of the present invention, an arc electrode structure is provided which allows the carbon nanostructures easily to be collected, and heat annealed. A first electrode has a central through bore therein. The through bore is connected to an outlet tube which, in turn, is connected to a collection box a pump. The pump draws the soot through the central bore and into the collection box so that soot is not deposited on the inner walls of the electrode chamber. In such a manner, the soot is easily, safely, and quickly collected. Further, as the soot is drawn through the central bore of the hot electrode, it is heat annealed, thereby perfecting the nanostructure. That is, as the soot travels along the central bore, the heat from the electrode allows a longer reaction time which produces longer SWNTs, and allows the removal of dangling bonds on the nanostructure.
Alternatively, instead of using the first electrode""s central through bore to remove soot from the electrode chamber, the central through bore can be used to introduce organic vapor, gas (including inert gas), and catalyst into the electrode chamber. That is, the apparatus of the present invention in an embodiment may be used to build carbon nanostructures, by CVD, from gaseous raw materials instead of from breaking apart carbon electrodes. Thus, by selecting the gases introduced to the arc-plasma region, the type and size of the carbon nanostructures easily can be controlled. Because the gases are introduced through the central bore in one of the electrodes, they are preheated before reaching the arc-plasma region, thereby increasing the yield of carbon nanostructures. Similarly, the introduction of gases through the electrode cools the electrode, thereby increasing safety and the electrode""s useful life. Further, because the gases are introduced through the center of the electrode, and the arc-plasma region is located above the central through bore, the gases must pass through the arc-plasma region, and a greater portion of the arc-plasma region is used. By introducing the organic vapor in such a manner, the amount of unused gas is reduced, which, in turn, reduces the cost of producing carbon nanostructures.
Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Invention and the figures.